Magnetic recording media consist of a base film or support covered with a magnetic coating. The magnetic coating is a hardened dispersion of particles of a magnetizable mineral in a polyurethane resin composition. Small amounts of other additives such as lubricants, dispersants, conductive agents, and the like (further specified below) also form part of the magnetic coating composition.
Most present magnetic coatings require crosslinking after they are applied to the support. This is usually accomplished by adding a polyfunctional isocyanate crosslinking agent to the magnetic coating before deposition on the support. After deposition and drying to remove the solvent, the tape is wound or otherwise stored, then aged for one or two days to allow crosslinking to take place. An overview of the ingredients, physical and chemical properties, and manufacture of conventional magnetic media can be found in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 14, pages 732-753, John Wiley and Sons (New York, 1981).
There are two important disadvantages to the above stated method of manufacturing magnetic media. First, the chemical crosslinking reaction between the polyurethane resin and polyfunctional isocyanates is considered to be the most critical and difficult step to control during the production of a magnetic medium. This difficulty arises because the solvents, magnetizable mineral, and ambient atmosphere each contain variable amounts of water. The reaction of water with the isocyanate moieties of the crosslinking agent is favored over the reaction of isocyanates with organic hydroxyl groups to crosslink the coating. As a result of the interfering reaction with water, crosslinking often is not complete and the resulting magnetic tape coating can be of poor quality.
A collateral disadvantage of chemical crosslinking is that the medium must be stored for a substantial length of time to complete the crosslinking reaction. The tape must be stored under very carefully controlled conditions to prevent it from blocking during the crosslinking reaction.
One solution to the problems of chemical crosslinking has been to devise a magnetic coating which does not require crosslinking in order to provide acceptable properties. U.S. Pat. No. 4,284,750, issued to the present inventor on Aug. 18, 1981, describes such a composition.
Another solution proposed to resolve the chemical crosslinking problem has been to provide polyurethane compositions which can be induced to crosslink using actinic radiation, and particularly electron beam radiation. Two patents suggesting electron beam crosslinkable polyurethane tape coatings are U.S. Pat. No. 4,408,020, issued to Kolycheck on Oct. 4, 1983, and U.S. Pat. No. 4,415,630, issued to Kubota, et al. on Nov. 15, 1983.
Electron beam or radiation curable polyurethane compositions are suggested for other utilities in the following patents:
______________________________________ U.S. Pat. No. Inventor Issued ______________________________________ Re 29,131 Smith, et al Feb. 2, 1977 3,654,347 Kincaid, et al Apr. 4, 1972 3,719,638 Huemmer, et al March 6, 1973 4,264,757 Park Apr. 28, 1981 4,399,239 Herwig, et al Aug. 16, 1983 ______________________________________
Various chemically cured polyurethanes are taught in the following patents:
______________________________________ U.S. Pat. No. Inventory Issues ______________________________________ 3,779,995 Dannels, et al Dec. 18, 1973 4,203,875 Garner, et al May 20, 1980 4,400,497 Blum, et al Aug. 23, 1983 ______________________________________
The compositions disclosed in the listed patents generally lack one or more of the essential properties of a binder for the high quality magnetic tape coatings now required. First, to minimize friction between the ultimate recording medium and the recording or playback heads, the binder must have a yield point of greater than 3,000 psi (2069 N/m.sup.2) and a Young's Modulus of greater than 50,000 psi (34,500 N/m.sup.2) at temperatures between about 20.degree. and 80.degree. Celsius. Second, the binder should have a high ultimate strength and hardness, and should be capable of elongating at least about 130% before breaking. To provide these properties, the finished coating should be highly crosslinked, meaning that it should have a high gel fraction. The resinous portion of the coating should have a weight average molecular weight of at least 80,000, a glass transition temperature (T.sub.g) of at least 30.degree. Celsius when crosslinked, and the crosslinked magnetic coating should have a T.sub.g so high that it is never exceeded during ordinary use of the magnetic medium. The finished tape should have good resistance to blocking. Additionally, the uncured magnetic coating should be capable of being applied with conventional coating apparatus (which requires that the coating be rapidly dried), contain a high loading of well-dispersed magnetizable mineral, and be capable of being calendared before the coating is crosslinked. The coating should have a viscosity of roughly 1,000 to 5,000 cps, whether containing a reactive or nonreactive solvent. Furthermore, the coatings should not shrink substantially upon drying, and must be capable of adhering both to the magnetizable coating and to the substrate.
Prior electron beam curable compositions have not been capable of providing recording media having all the required properties. It is thus one principal object of this invention to provide an electron beam curable magnetic tape coating, binder, and finished magnetic tape which are easily processed and provide all the necessary chemical and physical properties.